1. Field of the Invention
The present invention relates to a voltage controlled oscillator (VCO) and more particularly to a voltage controlled oscillator utilizing an LC type resonant tank.
2. Description of the Prior Art
Voltage controlled oscillators (VCO's) find broad application in electronic systems where voltage-to-frequency conversion is desired and are particularly useful in phase-locked loop circuits (PLL). A voltage controlled oscillator generates an AC output signal V.sub.o whose frequency, f.sub.o, changes in response to a control voltage signal V.sub.c applied at an input terminal.
The output frequency f.sub.o is inversely related to the period between alternating cycles of the output signal V.sub.o. A variable timing element is generally used to change the period between alternating cycles and thus change the output frequency. Typically, the variable element is either a variable capacitance diode C.sub.v (varactor) or a variable resistance device R.sub.v (see FIGS. 1A and 1B). Known devices in either category have intrinsic limitations with respect to thermal stability, linearity, adaptability to integrated circuit techniques, etc., that complicate the design of VCO circuitry.
If a VCO circuit is to be useful in a variety of environments, the output frequency f.sub.o should change linearly with the input control voltage. Linearity is optimally desired over a wide range of input voltages and a wide band of output frequencies. When the application environment is sensitive to noise at particular frequencies, it is desirable to be able to confine the harmonic content of the output signal V.sub.o to a specifiable frequency range.
Various factors prevent concurrent realization of these objectives in conventional VCO designs. For example, if a variable capacitance diode C.sub.v (varactor) is selected as the variable element that changes the frequency of the output signal, there are intrinsic non-linear characteristics associated with known variable capacitance diodes that must be compensated for. The requisite compensating circuitry prevents design of a simple VCO circuit with broad range linearity over a wide band of output frequencies and a wide range of input voltages. Known designs generally require switching among a plurality of variable capacitance diodes if broad range linearity is to be obtained. The switching circuitry adds to the cost of the VCO.
If on the other hand, a variable resistance element R.sub.v is chosen for altering the time delay T between output cycles (oscillation period), intrinsic thermal stability problems and current capacity limitations associated with known devices come into play. Some of these problems are discussed below.
Generally, two types of timing circuits, RC and LC, are used to determine the output frequency of a VCO. The classic RC circuit (FIG. 1A) has a resistor-capacitor combination coupled to a threshold detector. The product of R and C determine the amount of time needed for charging or discharging the capacitor to a predetermined voltage level V.sub.threshold. This charge/discharge time (also referred to as the time constant of the circuit) determines the period T of oscillation of the output signal.
RC circuits have no natural or central frequency at which they prefer to oscillate. Capacitor charge time can theoretically be varied over an infinite range which allows output over a wide range of frequencies. However, the wide frequency range is a mixed blessing, since noise in the input voltage of an RC type VCO can generate an output signal which includes harmonic noise components at undesired frequencies throughout the entire frequency spectrum. In many system designs, noise needs to be contained or confined so it does not interfere with sensitive frequency ranges of a particular environment. Such noise containment is not easily achieved when an RC type VCO design is used.
RC type VCO's are limited in other respects. When a simple resistor is used to charge and/or discharge the timing capacitor of an RC circuit, the circuit exhibits poor temperature stability due to the intrinsic thermal characteristics of known resistors. Power supply fluctuations pass through the resistor easily and change the charge time of the simple resistor circuit. Unpredictable changes in temperature and/or power supply voltages reduce the circuit's reliability.
Semiconductor devices with stable electrical characterics, known as band gap devices, may be used in place of conventional resistors to provide a far more stable current source for charging the timing capacitor. Band gap devices rely on the intrinsic energy band properties of the semiconductor material from which they are made. Their output remains constant over a wide range of temperature and power supply variations. The output current available from known band gap devices is unfortunately limited to very small values. If a band gap device is to be used for charging a timing capacitor C.sub.t to a desired voltage level V.sub.threshold in an RC type circuit (FIG. 1A) and the circuit design requires a high frequency output (a short charging period), the band gap device will not be able to supply sufficient current to charge the capacitor unless the capacitance of the timing capacitor is reduced to very small values (10 pF or less for example). Such low capacitance values are undesirable because stray capacitance effects in the circuit become significant. These stray capacitance are unpredictable and circuit reliability suffers as a consequence. The use of band gap devices is accordingly limited to low frequency RC type VCO's.
The second type of VCO circuit, known as the LC type circuit, uses a resonant circuit or an oscillation tank to determine output frequency (see FIG. 1B). The oscillation tank circuit typically includes an inductive element L and a capacitive element C.sub.v connected in series, in parallel, or in a series-parallel combination to form a resonant tank. A feedback circuit amplifies and returns a portion of an oscillating current or voltage within the tank back to the tank to keep that oscillation signal within from decaying. The oscillation tank has a natural or central frequency (f.sub.c =1/2.pi..sqroot.LC) at which internal oscillation signals will prefer to oscillate. Frequency shifting in known LC type VCO designs is generally achieved by varying the capacitance value, C, of a variable capacitance element C.sub.v in the LC tank. The new capacitance alters the natural frequency f.sub.c of the tank circuit. The LC type VCO is preferred over the RC type in noise sensitive environments because noise signals at the input end of an LC type VCO produce output noise harmonics that are contained or centralized near the center (resonant) frequency f.sub. c of the tank circuit. The RC type VCO, in contrast, can generate noise with harmonics spread over a wide spectral range. The preference of the LC circuit to operate at frequencies near its natural frequency is known as the Q of the circuit.
Referring to the frequency versus magnitude graph of FIG. 2, signals in a circuit with high Q tend to oscillate at frequencies very close to the natural or central frequency f.sub.c. The magnitude of harmonics outside that range is generally negligible. Low Q circuits produce harmonics of significant magnitude that are spread over a wider range of frequencies. Noise generated within a low Q circuit can therefore spread further away from the resonant frequency f.sub.c and interfere with adjacent noise sensitive frequency bands. RC type circuits have no Q, and their output signals can therefore spread over the entire frequency spectrum. The Q of a tank circuit is determined by the respective values of its inductive and capacitive elements as well as any resistive elements in the resonant circuit. If these circuit parameters change during operation of the circuit, it becomes difficult to prevent undesired interference with adjacent noise sensitive bands.
In the conventional LC type VCO circuit, a variable capacitance element C.sub.v such as a varactor diode is typically used to shift the natural or central frequency F.sub.c of an LC tank circuit and this in turn changes the frequency of the output signal Vo. When the control voltage across the varactor is varied, either intentionally or because of a noise spike, the capacitance in the tank circuit is changed, and the Q of the circuit also changes. For low frequency environments this is generally not a problem because changes in Q are acceptable in most systems. In high frequency systems, even small changes in Q may be significant because small changes in Q affect a larger range of frequencies. It is therefore undesirable to have any change in Q for such system environments, particularly when the VCO output frequency range lies close to noise sensitive frequency bands. A constant Q VCO which can confine noise harmonics to a fixed frequency range would be quite useful.
In addition to their inability to maintain constant Q, prior art LC type VCO's do not generally provide linear voltage-to-frequency conversion over a wide range of output frequencies. Known variable capacitance elements (varactors) have intrinsic bias voltage versus capacitance characteristics that are generally nonlinear. Referring to FIG. 3, the input voltage V.sub.c to output frequency f.sub.o conversion characteristics of a typical VCO utilizing a nonlinear varactor for frequency shifting is illustrated. The illustrated voltage to frequency conversion function is non-linear in general and includes a linear portion denoted as region L in which the output frequency f.sub.o is linearly related to the input control voltage V.sub.c. The region between the lowest frequency f.sub.L and the highest frequency f.sub.H of region L is termed the linear band of the VCO. When linear operation is required, the control voltage V.sub.c is limited to the linear input range between V.sub.L and V.sub.H. It is often desirable to have a VCO whose voltage to frequency conversion function is linear over a wide range of output frequencies and input voltages. Unfortunately, known variable capacitance elements that are available for use in conventional LC type VCO's have intrinsic non-linear characteristics that complicate design of such linear circuit.
Another design problem occurs with respect to circuit packaging. When variable capacitance VCO circuits are fabricated on integrated circuit chips, the varactor is typically formed as a separate element external of the VCO circuit chip. Varactors generally require large surface areas to achieve any useful capacitive action. If they were placed on the same chip as the rest of the VCO circuit they would waste valuable substrate area which could be put to better economic use. Off-chip varactors require external circuitry (off-chip support circuitry) for biasing and temperature compensation which adds to circuit cost and complexity. A conventional LC type VCO circuit that uses a variable capacitance element is therefore not easily minaturized because it is economically undesirable to place a varactor on the same chip with the rest of the VCO but it is also undesirable to provide off-chip support circuitry. RC circuits are in general more readily miniaturized by integrated circuit techniques, but they have other shortcomings such as their inability to provide noise containment in the frequency domain.
In summary, circuit designers face a number of considerations including circuit packaging (circuit integration), input voltage versus output frequency linearity, containment of output noise within a specified frequency range, thermal stability assurance and providing immunity to power supply fluctuations, that present problems for either the RC or LC type VCO. RC type circuits are easily integrated on IC's but they suffer from thermal instability, poor immunity to power supply changes and they cannot contain noise within a specified frequency range. Previously known LC circuits have poor linearity and are not easily minaturized onto integrated circuit chips (IC's) because they use a non-linear variable capacitance element to shift their output frequency.